One common interconnect mechanism for transporting electronic signals on printed circuit boards, multichip modules and integrated circuits is a uniform width microstrip line or a uniform width stripline, as illustrated in FIGS. 1A and 1B. Such lines include a very low resistance signal transport line or “trace.” One or two signal return planes, and an insulating dielectric material separating such lines from the return planes, are illustrated in FIGS. 1A and 1B. Two adjacent signal transport lines can be positioned adjacent (but not contiguous) and parallel to each other to provide coupled differential signal pairs, as illustrated in FIGS. 1C and 1D. For a high speed transmission line, having a broad “flat” bandwidth region, control of line impedance within the bandwidth region is crucial. Most signal integrity problems arise from improper impedance matching of signal drivers and receivers to a signal interconnect.
Where a uniform transmission line is used, several groups of parameters are available for controlling the characteristic impedance of the line. For example, FIGS. 1A-1D illustrate the following groups of parameters: (1) thicknesses, h1 (FIGS. 1B, 1C, 1D), h2 (FIGS. 1B, 1C, 1D), and h3 (FIG. 1D) of the dielectric layers; (2) width w of the trace as shown in FIG. 1A; (3) dielectric permittivity c of the insulating substrate; (4) thickness h(tr) of the trace as shown in FIGS. 1B, 1C, & 1D; and (5) electrical conductivity a of the trace. Commercial requirements often limit the choices of most or all of these parameters. For example, most printed circuit boards (PCBs) are made of FR4 material so that one has little control of the dielectric constant of the insulating substrate or of the frequency dependence of the dielectric constant. Further, constraints on PCB packaging may severely limit the board thickness. Further, uniformly spaced traces, and traces spaced too close to each other, lead to signal cross-talk and other signal interference problems that limit bit rate and produce noise.
What is needed is an approach that provides additional parameters that allow control of impedance and/or signal interference of the transmission line. Preferably, this approach should not require that the traces be spaced far apart and should allow some freedom in control of frequency dependence of the trace of the trace impedance.